The present invention relates to a novel device for reducing the fouling of tubes in which a fluid flows, particularly tubes of a heat exchanger, of the type comprising a rotating turbulence-generating element, and used in industry, the petroleum or petrochemical industry in particular.
For some 20 years, devices have been known, installed so as to rotate freely inside tubes of heat exchangers, for the purpose of preventing the potential fouling due, for example, to impurities in suspension or to deposits of inorganic salts dissolved in the fluid, to coke undergoing formation in a thermal cracking process, or to sulfur-containing species soluble in hydrocarbons. Said impurities or said coke which, in certain temperature and pressure conditions, tend to deposit on the inside walls of the lines conveying the fluid, thereby cause flow obstructions which are detrimental to the operation of the process located downstream or, more seriously, hot spots on the inside surface of the tube, possibly leading to damage to its metal structure and thereby causing leaks of products.
Such devices are described for example in applications EP 0 174 254, EP 0 233 092, FR 2 637 659, EP 0 282 406, EP 0 369 851 and EP 1 227 292 filed by the Applicant. They have in common a mobile element, generally helix shaped, fixed to the upstream end of the tube so as to rotate freely therein under the action of the fluid flow. In the devices described in the abovementioned documents, the dimensions of the mobile element and of the interior of the tube are such that the mobile element does not scrape the inside wall of the tube continuously, in order to avoid any harmful effects on the service life of the tube and/or of the mobile element. The effect of reducing the fouling of the tubes is mainly due to the increase in the turbulence of the fluid, which prevents the formation of hot spots and the formation of deposits, thanks to a thermal homogenization effect, and also to a high shear effect close to the wall, which serves to detach deposits that are weakly attached to said wall.
In application EP 0 369 851, the Applicant further describes a mobile element comprising, in all its cross sections, at least one sharp edge shaped specifically so as to scrape the inside surface of the tube.
The only vulnerable point of the prior art devices is their fastening system, and, more precisely, the trunnion whereby the mobile element is retained in its fixed portion serving for fastening the device to the upstream end of the tube. In fact, the mobile element, which may be several meters long, and which is rotated by the fluid, is generally several scores of times greater and heavier than the trunnion, thereby exerting thereon a considerable tensile force. This tensile force, combined with high speeds of rotation of about 500 to 2000 revolutions per minute, exposes the fastening system to high friction, causing rapid wear of the moving parts.
The fastening systems used today in refinery heat exchangers have an average service life of between 12 and 36 months. It is obviously desirable to lengthen this period, because the replacement of these systems demands the opening of the heat exchanger, possibly requiring a complete shutdown of the installation in which it operates, and automatically incurring expenses and a loss of income to the operator.